It is well known that animals with marked reductions in nephron mass may maintain reasonably normal acid-base equilbrium under basal circumstances but have a diminished capacity to respond to acid-base challenges such as an acute mineral acid load. We know of no comparable information regarding the ability of the diseased kidney to respond to persistent alterations of carbon dioxide tension. This information would be of considerable physiologic interest as well as providing potential insights into certain mixed metabolic and respiratory acid-base disturbances encountered clinically. We proposed to produce chronic renal insufficiency by sub-total nephrectomy in the rat. GFR will be estimated by changes in serum urea and creatinine concentrations. Graded degrees of chronic hypercapnia and of chronic hypocapnia will be induced in these animals in the usual fashion utilizing our environmental chamber. The time-course of the renal adaptation, the pattern of urinary electrolyte excretion and the quantitative relationship between changes in plasma bicarbonate and hydrogen ion concentration, on the one hand, and steady-state change in carbon dioxide tension on the other, will be assessed as a function in the decrement in GFR.